1. Technical Field
This invention relates to the technology of making diamond crystals, and more particularly to the art of making diamond films of such crystals in a free-standing condition.
2. Discussion of the Prior Art
The growth of diamond crystals, particularly thin films, from a vapor phase has been tried according to several types of methods as delineated in U.S. Pat. No. 4,740,263. One of the most effective methods is that of chemical vapor deposition which comprises activating a gaseous mixture of a carbon containing gas mixture, usually methane and hydrogen, the activation being by a variety of mechanisms such as microwave discharge (see U.S. Pat. No. 4,434,188), by dual ion beam activation of a methane/argon mixture (see U.S. Pat. No. 4,490,229), by use of a hot filament which may be comprised of tungsten (see U.S. Pat. No. 4,740,263), or by any variety of thermal techniques using heat, electron beam, light, DC discharge, AC glow discharge, DC arc discharge, to excite a gas containing an organic compound (see U.S. Pat. No. 4,816,286). The substrates for deposition of diamond films have usually been quartz and silicon. None of these techniques directly produce a free-standing diamond film because of its adherence to the substrate which is important to the nucleation process of the diamond film.
To obtain a free-standing diamond film, the prior art has conventionally turned to slicing of bulk diamond crystals (natural or synthetic) which has proved to be costly and difficult, especially for thin films. The mechanical stress of such cutting as well as the induced thermal effects, cause stress which leads to cracking or shattering of the diamond crystals.
Another technique used by the prior art to obtain a free-standing film is that of extensive chemical/physical etching of silicon substrates after the chemical vapor deposition process is completed. Essentially, the film and substrate are tipped over and the substrate removed by application of a strong acid which etches away the silicon substrate over an extremely long period of time. This process is complicated and time-consuming, and subjects the diamond film to thermal stress which leads to cracking or shattering.
Although the above represents known techniques for removing diamond from substrates upon which they have been deposited, there has been some attempt by the prior art to remove substrates from nondiamond films. In U.S. Pat. No. 4,250,148, silicon ribbons, deposited on metal foil, were separated by stress developed after cooling; in U.S. Pat. No. 4,537,651, a germanium semiconductor material was deposited on a salt (sodium chloride) substrate, which substrate was melted by an electron beam and the molten salt drawn away by the capillary action of another wettable material (such as a tungsten support). Such techniques for nondiamond films have traditionally held out little hope with respect to diamond films which must be deposited on substrates which withstand chemical vapor at the extremely high temperatures of the deposition process itself and therefore are not readily removed at temperatures below which diamond is converted to graphite (1000.degree.-1200.degree. C.).
The invention is a method of fabricating free-standing diamond films for a large variety of applications, but is of particular interest to the automotive industry where ultra-thin diamond films may have potential use in finishes for chip resistance and in windshield coatings to prevent grit streaking from wipers. The method comprises (a) depositing and adhering diamond particles by hot filament chemical vapor deposition onto a substrate selected to be meltable at a temperature slightly in excess of the temperature of the substrate during deposition; and (b) prior to cooling said diamond particles, increasing the substrate temperature to melt at least a portion of the substrate while permitting such melt to emigrate from the diamond particles.
It is preferable to utilize a small diameter hot filament (about 0.0010") which is particularly comprised of tantalum or rhenium to facilitate such small diameter filaments; with such small diameter filament, deposition can be carried out at higher rates and with little destructive radiation effects upon the substrate or diamond film, the rates being in the range of 2-5 microns per hour.
Preferably, the parameters of the hot filament chemical vapor deposition comprise evacuating the deposition chamber to 1-100 Torr, activating the filament by an AC current without electrical bias to a temperature greater than 1900.degree. C., separately but simultaneously heating the substrate to a temperature in the range of 600.degree.-950.degree. C., flowing a carbon carrying gas mixture through the chamber at a rate of about 100-200 sccm, selecting the carbon carrying gas mixture to consist of virtually any hydrocarbon (typically methane, acetylene, or methanol) in combination with hydrogen gas, the hydrocarbon constituting 0.2-2% by volume of the gas mixture along with a limited amount of carbon monoxide to favorably suppress the formation of graphite (the CO being limited to restrict the oxygen/carbon ratio to 0.5-1.0), and, lastly, the time period for the chemical vapor deposition being adjusted to obtain the desired film thickness given the deposition determined by the conditions described.
Preferably, the substrate is a material that has a melting point about 50.degree.-300.degree. C. in excess of the deposition temperature during hot filament chemical vapor deposition. Such substrate is preferably selected from the group consisting of copper, gold, beryllium, manganese, Al/Fe, Al/Cu, and Ni/Sn, and nonmetals consisting of Al.sub.2 F.sub.3, CdF.sub.2, or CrF.sub.2. Preferably, the period for melting the substrate is in the range of five minutes to two hours and the melt is emigrated either by tilting the substrate and allowing it to drain by gravity or by absorbing the melt into a support screen therebelow.
A second aspect of this invention is to continuously form a ribbon of free-standing diamond film, the method comprising: (a) rotating a metallic drum constituted of a metal having a melting temperature greater than the melting temperature of the substrate for diamond deposition; (b) introducing molten substrate material to be carried by the rotating drum outer surface at a first position about the drum axis, the substrate material being meltable at a temperature slightly in excess of its temperature during deposition of the diamond crystals; (c) at a second location about such axis, where the substrate material has solidified thereon, depositing polycrystalline diamond particles by hot filament chemical vapor deposition while heating the drum in the region of such deposition to promote crystallization; and (d) at a third location about the axis, where the deposited polycrystalline diamond particles have merged to form a continuous film adhered to the solid substrate material, heating the substrate to at least its melting temperature, facilitating disadherence of the substrate material from the polycrystalline diamond film and permitting the diamond film to separate from the coated drum as a continuous diamond ribbon. If necessary, such continuous method may utilize a mechanical tongue to encourage the separation of the ribbon from the drum and the drum itself is contoured with a peripheral trough in its outer surface to contain the molten substrate material during the rotation of the drum. The surface tension inherent to the molten substrate material may be sufficient that it will remain as a liquid film on the drum, thus eliminating the need for collection and recirculation of the material.